1. Field of the Invention
This invention relates to the art of applying thin refractory coatings to a foam pattern for use in the evaporative casting process, and more particularly to the art of drying said thin refractory coatings with integrity and absence of imperfections.
2. Description of the Prior Art
Recently, the evaporative casting process (ECP) has been commercialized for use in making high volume automotive metal castings. It is a process in which polystyrene beads are expanded and fused to adopt the shape of a pattern mold. Both the product pattern and attendant gating is formed as an integrated unit or assembly. The pattern assembly is suspended within a flask followed by the injection of unbonded sand which is then vibrated to lock the sand grains about the pattern forming a completed mold. Hot molten metal is poured into the flask to thermally displace the polystyrene foam gating and product pattern. The foam is evaporated and its gaseous products migrate outwardly through the interstices of the dry sand.
In making automotive metal castings, many of the patterns are of a complex shape having hidden surfaces requiring that the pattern be formed of multiple parts which are glued together to form the completed pattern. The glue can often times be more heat sensitive than the foam pattern itself during the handling and coating processes. A refractory coating is necessary to improve the surface finish of the metal casting and to act as a temporary gas permeable mold surface.
It is desirable that such refractory coating be applied in a thin mode, typically by dipping the pattern assembly into a water suspension of the refractory particles. The coating thickness cannot be greater than 1/8 inch if such coating is to function as a porous temporary mold form. To maintain such thin coating, it is important that the aqueous suspension have a high water content; as much as 2 kilograms of water may be evaporated from each coated pattern. The water must be removed not only from the easily accessible outer surface of the pattern but also from the hidden under surfaces of a complex pattern. Use of heat to dry the thin coating cannot be used in an uncontrolled manner because the foam and glue joints are heat sensitive. Cool or warm air with or without microwave heating has been explored by the prior art.
Microwave energy coupled with flowing cool air, or simply flowing air by itself, has been used to dry relatively thick bodies of refractory material. In such cases, high levels of energy have been used because of the thickness of the body and the need for removal of a high amount of water. In U.S. Pat. Nos. 3,704,523 and 3,732,048, microwave energy was applied to wet molded ceramic objects with the simultaneous application of cool room temperature air over the mold. This early use of microwave energy in combination with a cool flow of air required an exorbitant amount of time to dry such object.
In U.S. Pat. Nos. 4,126,651, and 4,043,380, microwave energy was used in two stages to heat a solid plaster mold core to an internal temperature of about 300.degree. F., a temperature higher than the microwave heating temperatures (about 150.degree. F.) of the above discussed patents. Heating stages were separated by a room temperature air blowing step. The first stage of microwave energy heating caused the water in the thick plaster body to migrate to the surface, and the second stage drove the surface water away by evaporation. This method is inapplicable to solving the problem of flawlessly drying a thin refractory water coating on a heat sensitive foam pattern; it heats the body indiscriminantly to too high a temperature. The high water content of the plaster mold attracts so much microwave energy, even at lowered power levels, that the use of such method on a thin coating causes bubbles, cracks and the steam, resulting from such heating, melts foam and glue. To lower the power level, to reduce the heating temperature, would oxorbitantly increase the amount of time required for the drying procedure.
In U.S. Pat. No. 3,942,260, a thick refractory lining for a tundish or similar vessel was heated by microwave energy along with a simultaneous hot blast of air at a temperature of 300.degree.-400.degree. F. Again, the temperature attained would be destructive to the drying of a thin coating on a heat sensitive substrate.
A series of thin refractory coatings on shell molds have been used. In U.S. Pat. No. 3,850,224, only air drying was employed for each coating of the series, the air being applied at impact rates of 1000 feet per minute. Of course, such air drying at high impact rates would be destructive of the sensitive foam substrates under consideration here. In U.S. Pat. No. 4,180,918, the use of intermittent microwave energy along with cooling air at a temperature of about 55.degree. F. was employed to dry the multiple refractory layers for building up a shell mold on a wax pattern. Energy was applied for periods of about one minute, thus requiring a total time of about 5-6 minutes for each layer. There is no assurance that the use of this combination of intermittent microwave energy and cool air would in any way result in rapid production line cooling of a high water content thin coating for a foam pattern.
All of the above prior art fails to provide staged dehydration that can be carried out in a shortened period of time without harming the supporting foam pattern.